JP-2001-304018A (U.S. Pat. No. 6,591,183B2) shows an exhaust purifying system which includes sensors disposed upstream and downstream of a catalyst. The sensors respectively detect air-fuel ratio or rich/lean of exhaust gas. In such a system, a main feedback control is executed based on an output of the upstream sensor and a sub-feedback control is executed based on an output of the downstream sensor. In the main feedback control, a fuel injection quantity is feedback corrected so that air-fuel ratio upstream of the catalyst becomes a target air-fuel ratio. In the sub-feedback control, the fuel injection quantity or the target air fuel ratio is corrected, or a feedback quantity of the main feedback control is adjusted.
In order to restrict an increment of emission due to a deterioration of the catalyst, JP-2006-291773A discloses a system in which fuel injection quantity is alternately varied into rich or lean so that air fuel ratio of exhaust gas flowing into a catalyst is alternately varied to rich or lean. Thereby, a deterioration-determination of the catalyst is conducted based on a behavior of the output of the downstream sensor.
In this system, when a deterioration-determination of the catalyst is conducted during the sub-feedback control, the deterioration-determination becomes a closed-loop so that it is relatively difficult to compute a deterioration-determination parameter (for example, loss time, correlation coefficient, or oxygen occluded quantity). JP-2001-132434A (U.S. Pat. No. 6,470,674B1) shows a system in which a sub-feedback control is stopped in a deterioration-determination period, or a control constant of the sub-feedback control is varied to execute a deterioration-determination.
In the above system, the emissions are deteriorated in the deterioration-determination period. Even if a deterioration of the catalyst itself is accurately obtained, a deterioration of a purifying capacity of the entire exhaust gas purifying system including the catalyst and sensors cannot be determined.